National Climate Report: Q&A with Authors

November 3, 2017

Projected changes in average annual temperatures under different global warming scenarios. Source: Draft of the National Climate Assessment

By law, every four years Congress is provided with a state-of-the-art report on the impacts of climate change on the United States. The next National Climate Assessment is scheduled for 2018, but its scientific findings were published today. The massive document was produced by 13 federal agencies including NASA, the Army Corps of Engineers and the Environmental Protection Agency, along with top university scientists, and approved by the National Academy of Sciences. Four of its 32 lead authors are affiliated with Columbia University; two of them speak below.

Among the report’s conclusions:

-Since 2014, evidence that humans are causing rapid warming of the atmosphere and oceans has only strengthened. Many lines of evidence abound “from the top of the atmosphere to the depths of the oceans,” the draft says. “There are no convincing alternative explanations.”

-Average temperatures in the Lower 48 states increased 1.2 degrees F from 1986 to 2016, and are projected with very high confidence to rise faster in coming decades.

-Scientists have made significant progress in linking human influence to specific extreme weather events. Some extremes, including droughts, heat waves and huge rainfalls, have already become “more frequent, intense, or of longer duration, and many [are] expected to continue to increase or worsen.”

-The report increases the worst-case projection for global sea-level rise by 2100 to 8.5 feet, up from 6.6 feet in the last report.

-With rising sea level, tidal floods in some coastal cities have increased 5- to 10-fold since the 1960s, and continue to rise. The rate of increased flooding along the East Coast is accelerating.

-The rate of ocean acidification due to increasing carbon dioxide entering the water “is unparalleled in at least the past 66 million years.”

-Alaska, warming twice as fast as the global average, is “on the front lines,” with fast-wasting glaciers and “crumbling buildings, road and bridges and eroding shorelines.”

Radley Hortonis an associate research professor at Lamont-Doherty Earth Observatory. Timothy Hallis an adjunct professor at Columbia Engineering, and a senior scientist at the Columbia-affiliated NASA Goddard Institute for Space Studies.

What is new about this latest report?Radley Horton: With additional years of observations, and in many cases accelerating trends, the evidence is stronger than ever. The report reaffirms what over 97 percent of climate scientists have known for decades: the climate is changing, and humans are largely responsible. The risks to things we care about–the health of our children, our economic viability, the existence of our coastal cities–are real and growing. U.S. citizens have a right to know what risk they face.Timothy Hall: Since 2014, the planet has experienced its warmest years on record. This renders moot previous talk about a climate-change “pause,” which was never very compelling anyway. Data continues to accumulate that shows increased rates of extreme rainfall, one of the most robust climate-change predictions. Trends in hurricane activity are beginning to pop out of the noise. Regions of peak hurricane intensity have expanded poleward in concert with expanding regions of warm seawater.

At this point, can we say specific extreme events are related to climate change?Hall: With increasing computer power, so-called attribution studies are becoming more common and more reliable. We can now simulate an extreme event many times with and without human climate change, and say what fraction of the event’s hazard–the odds of the event occurring, rain amount, flood level, wind speed, etcetera–can be attributed to climate change. For example, attribution studies show that hurricanes Iselle, Julio and Ana, which hit Hawaii in 2014, were much more likely in a warming climate. We finished working on the report before the 2017 hurricane season. But climate change clearly played a role in Harvey’s rain-driven damage, and possibly a role in Maria’s wind-driven damage.

Could it be that we are in for bad surprises?
Horton: This is one of the most important points the report makes. While climate models incorporate many important processes, they cannot include all the possible changes that can contribute to tipping points and irreversible changes. These would be things like accelerated melting of the Greenland and Antarctic ice sheets, and resulting rapid sea-level rise. Or, extreme events like droughts and wildfires occurring in multiple places at the same time, or sequentially in one place. Such tipping points and compound extreme events can greatly multiply the systemic risks to society.

What is the likely damage? Will we be able to stem it?
Hall: Damage is already easy to see: more frequent events like Hurricane Sandy‘s storm surge, and Harvey’s rain flooding. More flooding like what Miami sees under clear skies. We can just sit and respond haphazardly after each crisis–more suffering and unplanned displacement–or we can adapt with some better long-term planning.Horton: Paradoxically, there is reason for optimism. Just like it’s harder to dismiss climate change, it’s harder to dismiss the idea that we could see rapid societal transition away from a carbon-centric economy. Global CO2 emissions appear to have been level over the past couple of years, during a period of economic growth. The cost of renewable fuels such as wind and solar have decreased dramatically. This is sending powerful signals to long-term investors and businesses about which way the wind is blowing. More and more businesses are asking the question: what risks do we face if we do not plan for a changing climate?